Heating devices with temperature regulating properties are used in the automotive industry. Some automotive applications, such as self-regulating heaters for external mirrors, do not require flexibility. Such heaters can be found in U.S. Pat. No. 4,931,627 and U.S. Pat. No. 4,857,711, both commonly owned with the present application.
Self-regulating heaters of the type described are constructed on a polyester substrate that does not allow stretch and has only limited flexibility. These product characteristics allow screen-printed heaters and conductor traces to function properly in many applications. When used in non-flexing applications, such as mirror heaters, stretch and flexibility are not required. However, it is desirable to use self-regulating heaters also in applications requiring flexibility, such as seat heaters in automobiles. In such applications, stiffness is required for durability so that conductor traces comprising the heater do not stretch and do not pull apart. However, this same stiffness can make the seat uncomfortable by limiting the ability of the heater to conform to the user. Further, the stiffness of the polyester base generates a crinkling noise when the heater is flexed. Again, in applications such as automobile seats, noise from crinkling of the heater is objectionable. Further, a seat heater must breathe. Seat coverings are often made of breathable material or material having small pin holes therein. The padding or foam used in the seat is also provided with holes for breathing. Breathing is necessary to wick away moisture from occupants remaining in the seat for long periods of time. Further, some automobile seats are even provided with ventilation systems, including fans and/or air conditioning circuits. Therefore, the heater positioned as a layer near the upper surface of the seat must also breathe to provide proper ventilation and comfort to the occupant.
The parent applications of the present application have provided a flexible heater which has been very satisfactory when flexibility is required, for example, in an automotive application, such as an automotive seat. However, further refinements are desired to allow the material of the heater to better flex without any degradation in performance, particularly regarding heating capacity and uniformity of heating. In particular, it is desirable to provide maximum flexibility together with maximum heater surface area. Similarly, still further refinements are desired in providing improved stress relief, flexibility and ventilation.